The present invention relates to an apparatus and method for setting knife blade depth for microsurgical applications.
Various types of knives have heretofore been utilized in microsurgical applications, such as the surgical technique known as radial keratotomy. For example, a steel blade fragment is placed between the jaws of a holder and the blade depth is thereafter set with a sight gauge under microscopic observation. Although the steel blade is generally considered the sharpest blade, it has a tendency to tear on a cellular level during use in microsurgical applications. For this reason, a diamond or sapphire blade knife, which uses a micrometer to control the exposure of the blade in discreet increments, has become increasingly popular for use in microsurgical applications. Regardless of the type of knife utilized, however, the setting of the knife blade depth requires accurate measurement of the length of extension of the blade edge relative to the edge or feet of the blade holder.
In a micrometer knife, the micrometer handle is in operative engagement with either the blade or the feet of the knife. The blade depth, or length of extension of the blade relative to the feet of the knife, is set by rotating the micrometer handle. However, play in the micrometer frequently results in an undesirable deviation from the desired knife blade depth. A sight gauge has also been used to check the knife blade depth of a knife set by means of a micrometer handle.
The setting of knife blade depth has heretofore also involved the utilization of a sight gauge, such as the block gauge illustrated in FIG. 1A or the coin gauge illustrated in FIG. 1B. The sight gauges illustrated in FIGS. 1A and 1B are shown measuring the blade depth of a conventional diamond blade knife 12 comprising a blade holder 14 and a diamond tipped blade 16 having an outer edge 16a. The blade holder 14 has a micrometer handle 18 on one end thereof in operative engagement with the diamond tipped blade 16. Rotation of the micrometer 18 causes the blade 16 to move inward or outward from the base or feet 20 of the blade holder 14 on the opposite end of the knife 12 from micrometer 18. Once again, however, conventional knives have also been utilized wherein the blade is fixed and the micrometer is in operative engagement with the feet.
As illustrated in FIG. 1A, the block gauge consists of a substantially planar material having a tapered surface 22 on one edge thereof and a plurality of perpendicular gradient marks 24 corresponding to the degree of taper. Usage of the block gauge illustrated in FIG. 1A entails the placement of the feet 20 flush against the tapered edge 22 and reading of the corresponding gradient mark intersecting the tip or edge 16a of blade 16. If the tip or edge 16a of the blade 16 does not extend the desired distance outward from the feet 20, the handle of micrometer 18 is rotated until the edge 16a of blade 16 intersects the desired gradient mark reflecting the degree of extension.
Usage of a coin type sight gauge, as illustrated in FIG. 1B, entails the placement of the knife 12 within the groove 26 of a tray 28. An eccentrically machined lip 30 having gradient marks 24 is thereafter rotated until the edge or tip 16a of the knife 16 intersects the lip 30. The point at which the edge or tip 16a of the blade 16 intersects the lip 30 reflects the degree of extension of the blade 16 outward from the blade holder 14. Once again, however, blade 16 can be adjusted by means of micrometer 18 until edge 16a extends the desired distance outward from feet 20, as measured by the gauge. In an effort to increase the accuracy of blade depth settings, the measurement of the blade depth of a micrometer knife with a sight gauge has heretofore also been done under microscopic observation.
Regardless of the type of knife utilized in the microsurgical application, it is extremely important that the knife blade depth, or length of extension of the blade edge 16a relative to the feet or edge 20 of the blade holder 14, be set with precision. For example, in radial keratotomy surgery, a number of radial incisions are made in the cornea by placing the feet or edge 20 of the knife holder 14 against the periphery of the cornea. In order to achieve the desired predetermined incision depth, the knife blade depth must be accurately set so that the edge 16a of the blade 16 only protrudes outward from the blade holder 14 by a distance equivalent to the desired incision depth. That is, in radial keratotomy surgery, the knife 12 is placed in proximity to the patient's eye until the feet 20 rest atop the cornea 32, as illustrated in FIG. 1C. The blade 16 thereby cuts the cornea 32 to a depth D equivalent to the distance between the tip or edge 16a of the blade 16 and the edge or feet 20 of the blade holder 14. In a typical radial keratotomy surgery, the cornea has a depth of approximately 0.6 millimeters (mm) and the radial incision cuts through approximately ninety percent (90%) of the cornea, or to approximately a depth of 0.54 mm. The surgical knife must therefore be accurately set so that the edge 16a of the blade extends outward away from the edge 20 of the blade holder 14 precisely 0.54 mm. A deviation of any magnitude could result in dangerous perforations of the cornea and/or damage to the patient's vision.
Despite the importance of accurately setting the knife blade depth, play in the micrometer handle has resulted in undesirable deviation from the desired knife blade depth and the sight gauges which have heretofore been utilized have been found to be inherently unreliable. For example, the use of a sight gauge, such as the block gauge and coin gauge illustrated in FIG. 1A and FIG. 1B, respectively, may result in parallax error, perhaps as high as ten percent (10%). Such error occurs in reading the gauge due to the fact that the observer's eye and the edge or tip 16a of the blade 16 are not in a line perpendicular to the plane of the scale. Further, repeated contact of the blade 16 with the gauge may result in degradation and damage to the blade 16. Finally, degradation of the gauge materials may result in a gradient variability as high as thirty percent (30%).